There are many applications associated with the development of subsea oil and/or gas fields where the flowlines or pipelines must be “pigged” between subsea wells or templates and the product-receiving “host” facility. A “pig,” or pipeline inspection gauge, describes any of a variety of objects with sealing discs or full bore devices that are moved through a pipeline. Some types of pigs, known at times as “intelligent pigs,” can include a long train of modules that incorporate sensors to measure pipe wall thickness, or pipe joint weld integrity, for example.
During the precommissioning of a pipeline, filtered seawater is often injected into the pipeline to ensure it is completely filled to enable a hydrostatic pressure testing. Following the completion of a hydrostatic testing operation on a pipeline, it may be necessary to execute a dewatering operation to remove any water from the subsea pipeline before oil and or gas is allowed to flow through the pipeline.
In a dewatering operation, at least one dewatering pig is launched from one end of the pipeline and is propelled through the pipeline by pumping compressed gas behind the pig with respect to the direction of the pig's travel. As the pig travels through the pipeline, the pig displaces any liquid from the pipeline.
In deep water pipelines, compressed gas of high pressure and flow rate is required to move the pig through a pipeline due to the weight of the water column ahead of the pig, or the hydrostatic head. Thus, high pressure and volume flow compressors, compressed air boosters, or pumps are needed to drive a dewatering pig or pig train through pipelines having a high hydrostatic head. For example, in a subsea pipeline that extends below the surface to depths of 7000 feet of water, over 3000 pounds per square inch (“psi”) of hydrostatic head results within a water-filled pipeline.
Because compressed gas of high pressure and flow rate is required to move the pig through a pipeline to overcome the high hydrostatic head, large surface support vessels, boats, or other platforms are required to support the large equipment required to produce the pressure and volume of compressed gasses and that are pumped into and through the pipeline. These large surface support vessels, boats, or other platforms are expensive in terms of capital equipment, cost of mobilization to site, and the consumption of fuel.
Another disadvantage of the prior art methods of moving a pig through a pipeline is the length of large diameter conduit, coiled tubing, or hose necessary to transfer the gas or fluid from, for example, a platform on the surface to the pig that is to be propelled through a subsea pipeline. Oftentimes, the conduit, coiled tubing, or hose that is used to connect a pipeline to the pumping equipment on the platform becomes tangled or damaged by sea currents, making the dewatering process difficult.
There is a need for a method to move a pig through the pipeline without having to use large pumps or compressors to develop the required large volume of compressed gas to propel the pig. There is also a need for a method to move a pig through the pipeline that does not require large support vessels or platforms to hold such large pumps or compressors. Finally, there is a need for a method to move a pig through a pipeline that does not require a conduit, coiled tubing, or hose that is of great diameter and lengths.